Method for minimizing warp and die stress in the production of an electronic assembly

ABSTRACT

A method of minimizing warp and die stress in the production of an electronic assembly includes connecting one surface of a die to a package, and connecting an opposite surface of the die to a lid disposed over a constraining ring that is mounted to the package. The lid has a size, shape and coefficient of thermal expansion (CTE) selected to generate a bending moment that opposes bending moments resulting from connecting the die to the package.

FIELD OF THE INVENTION

The present invention relates generally to the field of microelectronicfabrication and assembly and, more specifically, to methods and deviceswhich reduce or eliminate bending moments and reduce stresses in anintegrated circuit chip/package system which result from mis-matchedcoefficients of thermal expansion (CTE) for the various systemcomponents and/or from adhesive curing during assembly. Morespecifically, the present invention relates to assembly techniques forstabilizing and forming semiconductor chips/package systems, by matchingthe CTEs of the various components, by providing differential CTEcontrol within the components, and/or by offsetting CTE-induced bendingmoments with nulling bending moments.

BACKGROUND OF THE INVENTION

The electronic microcircuit, or "chip," in which a large number ofelectrical circuit components are diffused, for example, onto thesurface of a 1 to 4 cm² chip of silicon or germanium, has become anintegral and indispensable part of our industrial technology. Theindustrial significance of this technology is so great that continuousefforts are being made to improve chip performance, reliability andservice life.

The delicate structure and very small size of these chips, however, havecreated unusually novel and difficult technical problems. Theseproblems, generally caused by physical phenomena that are well known ona macroscopic scale, e.g., coefficients of thermal expansion (CTE), heatdissipation, adhesive shrinkage, flexural moduli, and the likenevertheless create entirely new and often undesirable sets of effectswhen they manifest themselves in the microscopic domain of theelectronic chip.

Illustratively, a typical chip is mounted on and is electrically coupledto a supporting substrate. The substrate, in turn, is secured to aprinted circuit board. Thus, the substrate not only serves theintermediate function of coupling electrical signals taken fromconductors on the printed circuit board to the chip for processing, butalso takes output signals from the chip and applies these output signalsto other printed circuit board conductors for further processing.

Because a chip, when energized, generates a considerable amount of heat,which can be on the order of 50 to 100 watts emanating from a chip withan area of 1 to 4 cm², the CTE for the chip and that of its substratecan produce a number of very damaging effects. Controlling this heat,generated in so concentrated an area, in a manner that avoids chipfailure through overheating is a problem that has not yet been solved ina fully satisfactory way.

For instance, one source of thermally related chip failure resides inthe fact that the electrical characteristics of the circuit componentsthat are diffused or otherwise impressed on the chip can vary markedlywith changes in chip dimensions. These chip dimension changes caused,for example, by thermal expansion of the chip can make the expanded chipproduce useless and, perhaps, damaging output signals. These undesiredthermal expansion effects can also cause the central portion of a chip,secured at its margins to a substrate, to curve, bend or bow. Thiscurving frequently causes at least some of the electrical connectionsbetween the chip and its underlying substrate to separate and disconnectfrom each other. Largely for these reasons, chip performance isdegraded.

Unquestionably, this destruction of circuit continuity through a thermalexpansion induced electrical connection failure is a CTE consequencethat must be at least minimized if it can not be fully avoided.

Other destructive effects that are attributable to thermally inducedcurving include chip cracking and breaking. In this circumstance, chipscrack and break in large measure because tensile stresses areestablished in the outermost surface of the chips as the chips are bent.These stresses, should they exceed the fracture strength of the chip,will cause the chip to crack or break. Thermal effects are not limitedto the chip, but also appear in the substrate and in other chippackaging materials.

A substrate is a structure that is assembled by stacking together two tofifteen or more layers of substrate materials, at least two of theselayers being of different compositions. The materials from which each ofthese layers are formed tend to be quite diverse, some layers, forinstance, being metal (e.g., copper, nickel or gold), other illustrativelayers being an epoxy resin and glass compound. The CTE for theseindividual layers, each being considerably different, invite anuncontrolled bending or thermally induced substrate surface distortionthat is applied not only to the chip during circuit operation, but alsoto the substrate through the high temperatures that are required insubstrate manufacture.

Preferably, the substrate surfaces that support the chip and establishelectrical contact between the chip and the printed circuit board shouldbe "flat" in all conditions of operation. Indeed, if the substrateitself is not sufficiently "flat", it will prove impossible to establishelectrical contact between the chip and substrate.

There are, moreover, other ways in which the desired degree of substrateflatness can be destroyed, apart from CTE-related effects. One of thesenon-CTE related losses in flatness is found in the inability to controlcompletely individual layer thickness in the layer manufacturingprocess. These individual layer thickness variations depart fromstandard or preferred thicknesses not only among different productionlots, but also in different portions or areas of the same layer. Thesevariations in individual layer thickness can contribute to localizedvariations in the coefficient of thermal expansion (CTE) within asubstrate, which may contribute to warping of the substrate.

To establish some standard for judging flatness in these microscopiccircumstances, and thus to distinguish acceptable variations in flatnessfrom those that are industrially not acceptable, several criteria havebeen established. First, "flatness" for the purpose of chip mounting andpackaging has been defined as the ratio of the maximum high to lowdeviation per unit area and has developed into an industry practice inwhich the maximum acceptable deviation from flatness is 2.5 μm. Further,there are other industrially accepted standards for warpage, or loss inflatness for an entire chip package and the chip package components, inwhich warpage of more than 6 to 8 mils over the entire chip package isindustrially unacceptable. This is a goal that is difficult to achieve,but it is a goal nevertheless, that the chip packaging industry mustmeet, in spite of the fact that thickness deviations in a givensubstrate layer can be as much as ±15%.

In an attempt to solve or at least to cope with these curvature orbending problems, the chip packaging industry has moved in two entirelyopposite directions. Ceramic substrates of about 40 mils or greater inthickness have been used. These thick substrates are so massive,relative to the supported chip, that chip bending does not occur.

The other, opposite, industrial approach has been to use substrates thatare essentially thin films, e.g., about 2 mils or less in thickness.These thin substrate films deform and absorb almost all of thecompressive stresses, surface irregularities and the like, thus leavingthe chip in an essentially flat, undeformed condition, similar to theway in which "blister" or "shrink-wrap" packaging conforms itself to theshape of the packaged item.

In passing, it also should be noted that there are sources of chipwarpage other than those described above. One illustrative non-thermallyor production related source of warpage is a consequence of the adhesiveunderfill that is applied between opposing surfaces of the chip and thecorresponding substrate area to secure the chip to the substrate and tostabilize the electrical connections between the chip and its substrate.The electrical connections in this substrate area directly under thechip, often referred to as the die area, usually are soldered joints. Inthis respect, the solder on these joints, over time, is subject to adeterioration that weakens and destroys the electrical connectionswithin the die area. This deterioration in the soldered joints has anumber of sources, one of these sources being a fatigue that is inducedthrough relative movement between the chip and its substrate. Althoughfilling the volume within the die area between the soldered joints withan adhesive (usually an epoxy resin cement) bonds the chip to thesubstrate and reduces relative movement as a source of the solderedjoint deterioration, the adhesive does produce some undesirable sideeffects. For instance, on curing or hardening, the adhesive becomes afurther locus of undesirable structural stress. The cured adhesiveshrinks, placing the soldered joints in compression and through thesecompressive forces thereby applies still another curving movement to thechip and the substrate.

Clearly, these techniques for coping, with greater or lesser success,with curving or bending moments, whatever their source, in massivesubstrates of about 40 mils or more in thickness or in thin filmsubstrates of about 2 mils or less nevertheless fail to suggest anysolution for corresponding problems among intermediate range substrateswith thicknesses greater than 2 mils and thinner than 40 mils.

Substrates in this intermediate range are too thin to force the chip tomaintain a suitable degree of flatness. These intermediate rangesubstrates are also too thick to absorb all of the bending moments,whatever the source, to enable the relatively thicker chip tostructurally dominate the combination and maintain an all-important chipflatness. It has been found, in fact, that substrates in thisintermediate range are excellent vehicles for transmitting bendingstresses to their respective chips, thereby aggravating the chip bendingstress difficulties summarized above. Nevertheless, in spite of thesestructural limitations, there is a significant commercial demand forchip substrates in this intermediate thickness range.

To complete a chip package, the chip usually is mounted in the center ofthe substrate. A ring to stiffen the chip and substrate combinationoften is bonded or secured to the substrate by means of an adhesive thatis applied to the margin of the substrate, essentially enclosing thechip within the ring's center either before or after mounting the chip.The ring forms a frame around the chip with the inner perimeter of thering being spaced from the corresponding edges of the chip and theheight of the ring being somewhat greater than that of the chip. In thisway, a lid, or cover, joined to that surface of the ring opposite to thering surface that is bonded to the substrate, is spaced above thecorresponding die area of the chip, leaving a gap between the uppersurface of the chip and the opposing, die area surface of the lid thatlater is filled with a thermally conducting material.

The CTE, adhesive shrinkage, flatness irregularities and other sourcesof warping, bending and distortion considered above apply withessentially equal force to the ring and to the lid. Accordingly, toproduce a marketable chip package these undesirable stress and bendingeffects, particularly for chips that have substrates with thicknessesgreater than 2 mils and thinner than 40 mils, and more particularlybetween 5 mils and 25 mils, should be avoided, or at least controlledand reduced in order to maintain chip and chip component flatness withinthe acceptable degree of flatness that is defined in relevant industrialpractice and standards.

SUMMARY OF THE INVENTION

The present invention relates to assembly techniques and the resultingproducts which are thermally stable, have high structural integrity, andcompensate for thermal stresses that occur between the variouscomponents of the package. This is accomplished, in-part, by designingthe package so that the coefficient of thermal expansion (CTE) of astiffening ring which is mounted on the package substrate matches theCTE of the substrate and optional lid. Further, the particular adhesivesused to bond the stiffening ring are chosen to match their CTE to thatof the substrate, ring and lid. Moreover, the substrate is designed sothat its CTE, at least in-part, matches that of the chip, and also thatof the stiffening ring.

I ADHESIVE ENCAPSULATION

For example, chip package bending can be significantly reduced byforming one or more slots or holes in the lid. This slot, or slots,establish fluid communication with the void space surrounding the chipthat is formed in the die area between the chip, the substrate, the lidand the margin between the edges of the chip and the inner walls of thering. An adhesive is injected through the hole into this void space. Inthis way, the chip is potted, or encapsulated, in the adhesivesimultaneously with being underfilled. Bonding the chip to the entiresupport structure effectively integrates the structure of the chip intothe more massive substrate, ring and lid combination. Thus, in a largemeasure, the bonded chip is protected from bending under any one or moreof the thermal and mechanical influences noted above.

Similarly, either independently of or in conjunction with the slot inthe lid, one or more slots or holes also can be formed in the substrate,the ring, or both, also to communicate with the void space thatsurrounds the chip. An adhesive introduced through a hole in thesubstrate or the ring alleviates the chip bending problem in the samemanner as the adhesive admitted through the slot in the lid. It has beenfound preferable, however, to flow the adhesive into the void space onlyfrom one or two slots. Flowing the adhesive into the void space frommore than two slots can produce bubbles in the adhesive, particularlyunder the chip, a most undesirable result.

II SELECTIVELY STACKED SUBSTRATE LAYERS

Turning now to the substrate, it has been mentioned that a substrate canbe formed by stacking from two to fifteen or more layers, at least twoof these layers each being of different materials. In this respect, thedominant layers affecting warping occur at the outer surfaces of thesubstrate. Through a salient feature of the invention, however, it hasbeen found that by using in the outer layers those materials over whichthe thickness and flatness can be most carefully controlled, e.g., alayer of copper, and adding, successively inward toward the center thoselayers that exhibit progressively greater production tolerance, warpingeffects can be markedly reduced, e.g., the warpage in one instance hasbeen reduced, through the practice of this feature of the invention fromabout 400 μm to under 150 μm.

By stacking the substrate layers in this manner, the overall warping ofthe substrate is avoided to provide a substrate from which theundesirable bending phenomenon has been generally eliminated, at leastwithin the range of temperatures that are ordinarily encountered in chipoperation.

This principle of the invention, moreover, has application not only tothe substrate, but also can be applied to the ring and lid structures,as appropriate.

III UNIT AREA COMPOSITION CONTROL

Recall that control of individual layer thickness can vary as much as±15% from the desired thickness. These thickness irregularities createfurther bending in the substrate, or other chip package components asdescribed above. In addition, the various layers may contain differingcompositions at different points in the layer. This might occur as aresult of patterning of a metal layer to form discrete conductivepathways. A chip package, manufactured in accordance with a process thatfurther characterizes the invention, however, will provide valuablereductions in bending stress. In accordance with this feature of theinvention, chip package components (of which the substrate isillustrative) are divided into small unit areas. The unit areacomposition in each layer progressively toward the outer surface of thecomponent is analyzed to determine if all of the portions of the layersthat are equal in distance from the plane of symmetry in the substrateunder each unit area contain essentially equal amounts of the samematerials. Respective layers are adjusted and controlled to produce achip package component in which quantities of the same materials thatare in each opposing layer within a unit area are approximately equal inamount, thereby providing a structure that has a generally low warpagethroughout.

IV DIE AREA CTE CONTROL

Through careful analysis of CTE mechanisms in the microcosm of chippackage technology, and in accordance with another feature of theinvention, it has been found from the standpoint of maintaining chipflatness through the range of expected operating temperatures, that theCTE of the substrate immediately under the die is very important. Ifthere is a CTE mismatch within the die area of the substrate and theoverlaying chip, undesirable and potentially destructive bending orelectrical contact shearing stresses will be applied to the chip andchip-substrate electrical connections. A salient feature of theinvention, however, overcomes this difficulty by approximating the CTEof the substrate die area to that of the chip, while matching theaverage CTE of the chip package, and more particularly, the average CTEof the substrate, to the CTE of the circuit board to which the packagedmicrochip is attached.

This feature of the invention reduces relative differences in CTEbetween the substrate and the chip, thereby avoiding the differential inthermal expansion that produces bending stresses in the chip, solderjoint shearing and fatigue. Thus, the chip and the die areas of thesubstrate, both enjoying generally the same CTE, expand and contracttogether as the temperatures change. By expanding and contractingtogether, relative movement between chip and die area of the substratethat in the prior art forced the chip to curve or to bend and appliedshearing forces to solder joints is eliminated. Further, by matching theaverage CTE of the substrate and the associated chip package to the CTEof the printed circuit board, relative movement and the concomitantbending stresses and shearing forces between the chip package and theprinted circuit board are also reduced.

V SELECTIVE CTE ADJUSTMENT

To further cope with bending stresses of thermal origin, the inventionalso provides for an unusual technique that selectively adjusts the CTEof one layer of material to approximate that of another layer. Astructure of this character has two or more grooves, recesses, or holes,of any predetermined and desired shape, formed in a matrix layer. Theseholes, or loci, are filled with another material that has a CTE which issignificantly different from the matrix layer CTE. On heating, thematerial in the holes expands at a rate and extent that is unlike thesurrounding matrix in which the loci are formed. Although thisdifferential expansion creates matrix layer stresses, at least in thevicinity of each of the loci, the aggregate effect of the expanded holefiller material, pressing against the surrounding portions of thematrix, increases the actual CTE of the matrix layer. Through anappropriate selection of the number and arrangement of the loci formedin the matrix and choice in filler material (or materials), withinlimits, the matrix layer or portions of that layer can be adjusted toproduce a predetermined CTE.

The opposite result also can be achieved through the practice of theinvention by fitting the matrix holes with an appropriate substance thatadheres to the surfaces of the holes. Thus, on application of atemperature appropriate to a degree in which the matrix hole fillingsubstance shrinks (in comparison with the matrix) the force applied bythe substance to the matrix contracts the matrix to a greater degreethan that of a matrix that has not been treated in accordance with theinvention. Consequently, the matrix that is so treated takes on CTEshrinkage characteristics that can differ markedly from the CTE of thebasic matrix material.

Applying this feature of the invention to the chip package, thesubstrate layers can be provided with a selected number and distributionof holes in the die area. These holes, filled with materials that havedifferent CTE's than the surrounding matrix, approximate the aggregateCTE of the die area matrix in order to approach the CTE of theoverlaying chip. Toward the periphery of the matrix, and under the ring,however, a different combination of matrix holes and filler materialsare chosen to enable the aggregate CTE for this portion of the matrixlayer to approach the CTE of the overlaying ring. Through properselection of the number of holes, their distribution in the layers andthe filler material for these holes it is now possible through thepractice of the invention to adjust the matrix CTE, and therebygenerally relieve both chip bending and bending stresses.

VI CHIP PACKAGE LID CTE ADJUSTMENT

It will be recalled, moreover, that the chip is a very concentratedsource of heat, a heat that must be dissipated if the chip is tocontinue to function properly or, in extreme situations, to function atall. A thermally conductive interface can be applied to the die areabetween the chip and the overlaying lid to conduct heat from the chip tothe die area of the lid in order to spread heat generated in the chipover a large portion of the surface area of the lid. A typical interfacesuitable for use with the invention is described in J. G. Ameen et al.U.S. Pat. No. 5,545,473 granted Aug. 13, 1996 and titled "ThermallyConductive Interface."

In those situations in which the chip is to be bonded to the die area ofthe lid, once more the stresses and bending effects imposed bydifferences between the chip CTE and the lid CTE become important.Ordinarily, chip package lids are formed from copper or aluminum.Alternatively, a combination of aluminum and silicon carbide or copperand silicon carbide or other low CTE reinforcement could be used. Inaccordance with another salient characteristic of the invention, it hasbeen noted, for example, that aluminum has a CTE of 23 PPM/°C.Consequently, by manipulating the ratio of aluminum to silicon carbidein different portions of the lid, any predetermined CTE in a spectrumthat extends from 23 PPM/°C. for pure aluminum to 3.7 PPM/°C. for puresilicon carbide can be prepared.

With this knowledge, a high silicon carbide and low aluminumconcentration composition can be established in the die area for the lidin order to match the CTE of the chip that is bonded to the adjoiningportion of the lid.

The marginal portions of the lid that are bonded to the ring, however,are of a different aluminum/silicon carbide proportions. In thisinstance, the relative concentrations of aluminum and silicon carbide inthe marginal portions of the lid are selected such that the average CTEof the lid matches the average CTE of the substrate/die combination.Through this technique, relative movement between the chip and the diearea of the lid, the ring and the portion of the lid that is bonded tothe ring and consequent bending as a function of thermal expansion isavoided, enabling each of the chip package components to remainessentially flat, while lowering stress on the die or adhesiveinterface.

The desired concentrations of aluminum and silicon carbide (to name justtwo of the possible materials) can be prepared in several ways. One ofthese techniques that also characterize the invention provides a porousshape with the same dimensions as the outside dimensions of the ring.The shape has a thickened central core of porous silicon carbide withdimensions about equal to the die area. One or more peripheral recessesat the margin of the shape establish a concentration of silicon carbidewhich matches, in part, the CTE of the ring. Molten aluminum isessentially dissolved in the porous silicon carbide matrix, the relativeproportions of aluminum and silicon carbide varying over the span of theshape to match the respective CTE of each of the underlaying components.In this way, the lid's peripheral CTE approximates a predetermined value(e.g., the CTE of the ring). Thus, through the practice of this featureof the invention, a lid is provided with very diverse thermal expansioncharacteristics, these characteristics matching the CTE of the lid diearea to that of the chip and the peripheral CTE of the lid to that ofthe ring.

VII CTE CANCELLATION

An additional feature of the invention counterbalances, or cancels, thebending moments that otherwise would be applied by the substrate to thechip, thereby eliminating relative movement between the substrate andthe chip and thus avoiding the associated bending of the chip that thisrelative movement would cause. In this characteristic embodiment of theinvention, electrically inactive components or passive electricalcomponents (e.g., capacitors, resistors and inductors) that have thesame CTE or a CTE that is similar to that of the active chip are coupledto the exposed die area surface of the substrate on the side of thesubstrate that is opposite to the side to which the electrically activechip is coupled. Because the electrically active chip and theelectrically passive or inactive elements both enjoy essentially thesame CTE, the thermal expansions of both chips are about equal. Relativemovements of these chips with respect to the substrate, although equal,are on opposite sides of the substrate, thereby effectively cancellingany chip-related bending moments that otherwise would occur. In thissituation, the electrically active chip remains suitably flat.

A further embodiment of this feature of the invention provides for theinsertion of a stiffener in the die area of the substrate to preventthat portion of the substrate from bending relative to the chip mounteddirectly over that portion of the substrate, through the range of deviceoperating temperatures. So mounted, the stiffener generally overcomesthe undesirable substrate warping in the chip die area.

Thus, in accordance with the principles of the invention, there isprovided method and apparatus for overcoming the potentially destructiveeffects of relative movements among chip package components. The scopeof the invention is limited, however, only through the claims appendedhereto.

BRIEF DESCRIPTION OF THE DRAWING

FIG. 1 is a plan view of a chip/package system embodying principles ofthe invention;

FIG. 2 is a sectional view of the chip/package system of FIG. 1, takenalong line II--II of FIG. 1;

FIG. 3 is a vertical sectional view of the package component of thechip/package system of FIG. 1;

FIG. 4 is a plan view of a portion of the package, showing a gridpattern for analyzing the material content within each grid of eachlayer of the package;

FIG. 5 is a horizontal sectional view, taken along line V--V of FIG. 3;

FIG. 6 is a horizontal sectional view, taken along line VI--VI of FIG.3;

FIG. 7 is a vertical sectional view of a layer used to form the package,and showing a plurality of pre-lamination holes;

FIG. 8 is a vertical sectional view of the perforated layer in apre-lamination stack with dielectric layers and outer conductive layers;

FIG. 9 is a vertical sectional view of the stack of layers of FIG. 8after lamination, in which material from the dielectric layers flowsinto and fills the plurality of holes;

FIG. 10 is a vertical sectional view of an alternative embodiment inwhich the layer is grooved instead of perforated, and filled with afiller material before lamination;

FIG. 11 is a plan view of a perforated layer, with the holes formed in adesired pattern;

FIG. 12 is a side elevational view of a preform used to manufacture alid having different regions of CTE;

FIG. 13 is a sectional view showing the preform in a mold for pressureinfiltrating molten metal into the ceramic preform of FIG. 12;

FIG. 14 is a side elevational view of the completed lid after molding;

FIG. 15 is a side elevational view of a lid having an opening in themiddle for receiving an insert having a different CTE than the remainderof the lid;

FIG. 16 is a vertical sectional view of the lid of FIG. 15, taken alongline XVI--XVI of FIG. 15;

FIG. 17 is a side elevational view of an insert used to complete the lidshown in FIGS. 15 and 16;

FIG. 18 is a sectional view showing a false die attached to an undersideof the package opposite the chip;

FIG. 19 is a scanning electron micrograph (SEM) image showing thenode-fibril infrastructure of an ePTFE matrix used in one embodiment ofthe present invention; and

FIG. 20 is a SEM image showing the node-fibril infrastructure of anePTFE matrix used in another embodiment of the present invention.

I ADHESIVE ENCAPSULATION

For a more complete understanding of the invention, attention isdirected to FIGS. 1 and 2, in which a chip/package system 10 includes apackage 12 having first and second opposite planar surfaces 14 and 16.An integrated circuit chip 18 is connectable through solder bumps orballs 20 to the surface 14 of the package 12. The solder balls 20establish both a mechanical and electrical connection of the package 12to the chip 18.

A constraining ring 22 having opposite surfaces 24 and 26 is bonded tothe package 12 through an adhesive layer 28. The constraining ring has acentrally located rectangular opening which defines a chip-mountingcavity 30. Generally, the constraining ring 22 stiffens the package 12to permit easier handling of the package prior to and after chipattachment and to reduce thermally-induced bending moments which resultwhen the coefficient of thermal expansion (CTE) of the package 12 ismis-matched with that of the chip 18. The thickness of the constrainingring 22 is slightly greater than the corresponding dimension of the chip18.

A lid 32 having opposite surfaces 34 and 36 is bonded to theconstraining ring 22 through an adhesive layer 38. The inner surface 38of the lid 32 is spaced from the upper surface of the chip 18. The outerperipheral edges of the lid 32, constraining ring 22, and package 12 areco-existent and form a square. In a particular embodiment, the squaremeasures 33 mm on each side. It is not necessary, however, for thepackage to be square shaped, although the square configuration is usedfrequently. Also, the 33 mm size is one embodiment, but other sizes canbe used.

When installed in an electronic device, the chip/package system 10 ismounted on a printed wiring board (PWB) 40 through solder balls 42. Aswith the solder balls 20, the solder balls 42 provide electrical andmechanical connection between the package 12 and the PWB 40.

Preferably, both the package 12 and the PWB 40 are made of multi-layeredlaminations of alternatingly disposed dielectric and conductive layers.The preferred dielectric materials, which are organic, as well as thepreferred conductive materials, will be discussed in greater detailbelow.

A problem in chip/package assembly arises when an adhesive underfill isapplied between the lower surface of the chip 18 and the upper surface14 of the package 12 to reinforce the mechanical connection between thepackage 12 and the chip 18. As the adhesive cures and shrinks, there isa tendency for the package 12 to warp, and since the chip 18 isconnected to the package 12, a bending moment can be applied to the chip18. This bending moment, if severe enough, can fracture the chip,disrupt circuits and components diffused in the chip, and/or compromisethe solder ball connections between chip and the package.

One aspect of the present invention is to provide a counter oroffsetting bending moment by filling the chip mounting cavity 30 with anadhesive which mechanically couples the lid 32 to the upper surface ofthe chip 18. The adhesive is introduced into the chip mounting cavity 30after the lid 32 has been bonded in place on the constraining ring 22.

The adhesive is applied in a liquid state through an opening 44, whichis preferably an elongated slot formed parallel to one of the four sidesof the chip/package system 10. As an example of an applicationtechnique, the liquid adhesive can be applied through a needle extendinginto the opening 44. The tip of the needle is juxtaposed within thesolder ball region and the underfill adhesive is released from the tipwhile advancing the tip along the slotted opening 44. The adhesivemigrates by capillary action to cover the area between the lower surfaceof the chip 18 and the upper surface of the package 12.

After the underfill adhesive is applied, the tip of the applicator orneedle can be juxtaposed within the space between the lower surface 38of the lid 32 and the upper surface of the chip 18, and as the needle isagain advanced along the slotted opening, capillary action draws theadhesive into this space.

The entire chip mounting cavity can be filled with the adhesive, asshown in FIG. 2, or simply the spaces between the chip 18 and surfacesof the lid 32 and package 12. As an alternative to the opening 44, or inaddition thereto, an opening 46 may be formed in the package 22. If theentire cavity is to be filled, there is no requirement for eitheropening 44 or 46 to be aligned with any one of the outer edges of thechip 18.

Opening 46 would likely be a hole, and not a slot. Moreover, twoopenings are generally preferred when filling the cavity with adhesiveto allow air to escape during adhesive filling. The two holes can bothbe formed in the lid, or one can be formed in the lid and the other inthe package, as depicted in FIG. 2.

Since the adhesive in contact with bond surfaces of the chip is curingsimultaneously, and since the adhesive couples the chip 18 to structureson opposite sides thereof, bending moments imparted by adhesiveshrinkage and CTE mismatch on one side of the chip are offset by bendingmoments imparted by heat shrinkage and CTE mismatch on the other side.

In conjunction with the offsetting or nulling bending moments, thepackage 12 and lid 32 are made of materials selected to approximatelymatch the CTE of the two components so that the opposing bending momentsare equal, but opposite.

The adhesive that is introduced into the cavity 30 is preferablythermally conductive to ensure that heat generated by the chip 18 passesfrom the cavity 30 to lid 32, which is likewise preferably made of athermally conductive material.

The adhesive layers 28 and 38 can be made of any suitable adhesivematerial, such as, but not limited to epoxy adhesives, porous substratesimpregnated with adhesives that are moisture resistant and are able towithstand temperatures in excess of 150° C., porous substrates that areimpregnated with an adhesive-filler mixture that are moisture resistantand able to withstand temperatures in excess of 150° C., preferably inexcess of 200° C., porous substrates that are impregnated with anadhesive-conductive particle mixture that is moisture resistant and ableto withstand temperatures in excess of 150° C., preferably in excess of200° C., e.g., Ablestik®. The adhesive layers 28 and 38 may be made ofthe same material or different materials. Several are commerciallyavailable, including the Ablestick ECF564 and ECF 564A conductiveepoxies, and GoreBond M6. The overfill and underfill adhesives can be aliquid epoxy containing SiO₂, e.g., Hysol 4526O. The overfill adhesivescan also be solders or other metallurgical bonding agents. Othersuitable materials for use as adhesive materials are described below.

The adhesive materials used for adhesive layers 28 and 38 are preferablyin sheet form, while the adhesive filled in the cavity 30 is preferablyin liquid form. However, the sheet adhesive layers could be replacedwith liquid materials, and the liquid materials in the cavity 30 couldbe replaced with sheet materials, at least with respect to the areabetween the chip 18 and lid 32.

II SELECTIVELY STACKED SUBSTRATE LAYERS

FIG. 3 is a vertical sectional view of a laminated structure for thepackage 12, which may be used in the chip/package system of FIG. 1. Inaccordance with a feature of the invention, it has been found that byusing in the outer layers 56 and 78, illustratively, each layer beingabout a 20 μm thick layer of metal, those materials that have the leastdeparture from the manufacturer's thickness tolerance (usually the metallayers) and by progressively adding from the outer layers 48 and 50toward the center or core layer 52 (e.g., copper, 35 μm thick) thosematerials in which manufacturing thickness tolerances becomeprogressively greater (usually the dielectric layers), the overallsubstrate is balanced from a thermal expansion standpoint.

The balance is such, moreover, that the curving, and its destructiveeffect on the chip (not shown in FIG. 3), or the ability to establishelectrical contact with the chips is largely overcome.

A combination of layers, to form a suitable substrate that does notexhibit significant thermal bending within the normal range of chipoperation, as shown in FIG. 3, would comprise, in order from thesoldermask layer 48, inwardly toward the conductive copper center ofcore layer 52 through which a plane of symmetry passes:

a) 20 μm Cu/Ni/Au layer 56, a conductor;

b) 44 μm cyanate ester-epoxy-ePTFE (CE/E-ePTFE) layer 58, a dielectric;

c) 9 μm Cu layer 60, a conductor;

d) 50 μm bismalimide-triazine (BT)-Epoxy/Glass layer 62, a dielectric;

e) 18 μm Cu layer 64, a conductor; and

f) 44 μm CE/E-ePTFE layer 66, a dielectric.

For example, the CE/E-epoxy-ePTFE layer 58 is prepared as follows: avarnish solution is made by mixing 5.95 pounds of M-30 (Ciba Geigy),4.07 pounds of RSL 1462 (Shell Resins, Inc.), 4.57 pounds of 2, 4,6-tribromophenyl-terminated tetrabromobisphenol A carbonate oligomer(BC-58) (Great Lakes Inc.), 136 g bisphenol A (Aldrich Company), 23.4 gIrganox 1010, 18.1 g of a 10% solution of Mn HEX-CEM in mineral spirits,and 8.40 kg MEK. The varnish solution was further diluted into twoseparate baths--20% (w/w) and 53.8% (w/w). The two varnish solutionswere poured into separate impregnation baths, and an e-PTFE web wassuccessively passed through each impregnation bath one immediately afterthe other. The varnish was constantly agitated so as to insureuniformity. The impregnated web was then immediately passed through aheated oven to remove all or nearly all the solvent and partially curethe adhesives, and was collected on a roll. The ePTFE web, such as thatshown in FIG. 11, may be any desired thickness, such as 25 μm, 40 μm,for example. A 25 μm thick material has a weight per area ofapproximately 11.2 to 13.8 g/m².

As shown in FIG. 3, an identical, corresponding array of layers isprovided between the outer layer 50 and the core layer 52. These includea dielectric layer 68, a conductive layer 70, a dielectric layer 72, aconductive layer 74, a dielectric layer 76, and a conductive layer 78.Note should be taken of the fact that copper layer 60 and correspondingcopper layer 74 on the opposite side of the symmetry plane 54 areinterspersed among the layers of CE/E-ePTFE (which has a manufacturingthickness tolerance considerably greater than the copper layers) andBT-epoxy/glass to meet the electrical needs of the apparatus, i.e., thecircuit (which has a manufacturing thickness tolerance greater thanCE/E-ePTFE) traces for the conductors and, as such, are not true copperlayers, but are a pattern of copper conductors. The principle of thisaspect of the invention, that the better controlled thickness materialsare positioned toward the outside of the package 12, and the layers withlower thickness control positioned closer to the core layer 52 must beviolated occasionally, as it can be seen with respect to the CE/E-ePTFElayer 66, to satisfy the primary electrical needs of the microchippackage.

Because the thicknesses of the copper layers 54, 70 and 64 can becarefully controlled and the BT-epoxy/glass and CE/E-ePTFE layerthicknesses can also be controlled, but not as well as the copper layerthicknesses, the interposition of these copper layers among the B-Tepoxy/glass and CE/E-ePTFE does not aggravate the CTE response of thesubstrate 12, but further serves to reduce the aggregate CTE of thesubstrate 12. The principal feature of this aspect of the invention,consequently, is characterized in the fact that the manufacturingtolerances, and thus, the potential for departing from a predeterminedlayer thickness and aggravating the overall CTE response of thesubstrate 12 is greater for the CE/E-ePTFE layer 66, which also has thehighest CTE of any of the layers, than it is for any of the other layermaterials in the substrate 12.

Other appropriate layer materials can be substituted for those which aredescribed in connection with FIG. 3, it only being necessary to arrangethe layers according to the principles expressed above. For example, FR4, a common dielectric material (or any suitable dielectric material)layer can be substituted for the BT-epoxy/glass layers shown in FIG. 3.Moreover, a greater or lesser number of layers forming the laminatedpackage 12 can be employed.

To illustrate more clearly the improvement in the mechanical propertiesof the substrate 11 through the practice of this aspect of theinvention, attention is invited to the following table:

    ______________________________________               Prior Art Construction                           FIG. 3 Construction    ______________________________________    Flexural Modulus                  6.5 to 10.3  19.4 to 26.    (GPa)    CTE (ppm/°C.)                 30.2 to 32.4  21.4 to 23.0    Warpage (μm)                 388           83    ______________________________________     .sup.1 GPa = 10.sup.9 pascals, in which one pascal is equal to one     newton/m.sup.2.

The compositions of the layers that form the substrate 12 can be changedthrough a substitution of other known substrate layer materials, asnoted above, it being clear that the improved performance that isprovided through the invention is attained by positioning those layerswith the greatest manufacturing thickness tolerances (usually thedielectrics) as the innermost layers in the substrate.

As evident from the above, the arrangement of individual layers in thelaminated substrate or package 12 is selectively made to achievedimensional control, increased flexural stiffness, and matching of CTEswithin the substrate. Thus, it is important that the layers on oppositesides of the plane of symmetry are made of the same material and havethe same thickness to avoid CTE differentials within the substrate. Notonly is it desirable to place the metal layers on the outer layers ofthe laminated substrate, but it is also desirable to make the outermetal layers thicker than the inner metal layers to enhance flexuralstiffness.

III UNIT AREA COMPOSITION CONTROL

Another aspect of the present invention is to analyze the materialcomposition of each layer to determine whether offsetting formations inopposing layers should be made to ensure that the CTEs of the two layersare matched.

For example, and referring to FIGS. 3 and 4, conductive layer 60 is apatterned layer that may have areas where metal has been removed to forma circuit. At an area of the layer 60 where metal is removed, acorresponding area of symmetrically disposed conductive layer 74 mayhave a different metal content, resulting in localized differences inCTE between the two layers 60 and 74. The differences in CTE can resultin warping. The same would be true for layers formed by deposition,where more or less metal is deposited on the different but symmetricallyopposed layers.

One aspect of the present invention is to analyze the material contentof one layer, and then alter the opposing layer to match the materialcontent. Referring to FIGS. 3 and 4, substrate 12 is divided into anarray of unit area squares 80. These squares measure, for example, 1mm². The squares 80 extend in space through all layers of the package sothat each square of one layer has a counterpart area in a symmetricallyopposed layer.

The compositions of each layer in the substrate 12 that are within theconfines of each of the respective squares in the array of squares areanalyzed to identify the materials and the concentrations of each ofthese materials in every layer of the unit area under consideration.

This concentration, or distribution of materials within each layer isdeveloped from the original data in the design files for the layer orthrough analysis of the mask work for the layer. For example, as shownin FIG. 3 and 4, the copper layers 60 and 74 are not, as mentionedabove, true layers, but are a distribution of conductors within thepackage 12 that establish appropriate conductors for the electricalconnectors. To better illustrate this situation, attention now isinvited to FIG. 5, which shows a plan view of the section V--V in FIG.3.

As shown in FIG. 5, the copper conductors that comprise the "layer" 60do not extend across the entire area of the substrate 12, but onlyoccupy a small non-uniform or unequally distributed portion of thecopper in that area, the BT-epoxy/glass layer 62 being exposed in thoseportions of the plan view area of the substrate 12 that are not coveredby the copper conductors which comprise the layer 60.

FIG. 6, in turn, shows the unequal or non-uniform disposition of thecopper electrical conductors 74, and the somewhat larger concentrationof copper in the conductors that form the layer 74 in comparison withrespect to the quantity of copper in the layer 60.

In this manner, the illustrative copper concentration per unit area inthe layers can be developed. As shown in FIG. 6, the adjustedconcentrations of copper in common unit areas 80 in each of the layers60 and 74 are not identical because of electrical circuit requirementsthat are imposed on these layers. The electrically inactive copper strip60A in the layer 60 and the electrically unnecessary copper portion 74Athat is removed from the layer 74 nevertheless do establish anapproximate balance between the two copper electrical circuit layers 60and 74. Warping can be reduced considerably in the substrate 12 bybalancing the concentrations of the same substrate layer materials.Balancing, for this purpose, is defined as establishing equalconcentrations, or void spaces on opposing pairs of layers that are atequal distances from the plane of symmetry.

For example, to balance the amount of copper in the layer 60 (FIG. 5) toapproximate the quantity of copper in the companion layer 74, a strip ofelectrically inactive copper 60A is added to the surface of theBT-epoxy/glass layer 62. Alternatively, the relative copperconcentrations also can be balanced by removing an equivalent amount ofelectrically unnecessary copper 74A from the electrical conductors thatform the layer 74.

Of course, perfect balance can not be achieved in actual practice.Balance, for the purpose of this invention, is reached to the extentthat the electrical requirements of the circuit are satisfied whilenevertheless establishing as close an approximation to an equalitybetween the respective copper concentrations in each of the layers asthe physical structure will permit.

Where an imbalance of bending moments exists between one opposed pair oflayers, e.g., layers 60 and 74, the imbalance may be compensated for bypurposely imbalancing a second pair of layers, e.g., 64 and 70, so as tocreate an equal and opposite bending moment.

Naturally, it is within the scope of the invention to strive for balanceby using any one or more of the three foregoing techniques toapproximate the desired goal.

Although a specific example of the invention is described in connectionwith the electrical conductor layers 60 and 74, balance, as defined forthe purpose of this invention, also should be sought among theconstituents that are incorporated in the Cu/Ni/Au layer 56, theCE/E-ePTFE layer 58 and the BT-epoxy/glass layer 62 through the processdescribed above with respect to copper.

In this way, by establishing a general balance, or equality, inconstituent concentrations among the layers that are within each of theunit area squares that are defined by the unit area squares 80, curvingand the otherwise undesirable CTE bending moments are overcome, to alarge extent.

This technique for manipulating the layer constituent materialsconcentration is not at all limited to the substrate 12, but can beapplied successfully to all or some of the other components ofchip/package system, including the lid and constraining ring.

An essential part of the foregoing is that the CTE of the opposinglayers is matched locally. Thus, when using the same materials foropposing layers in a laminated structure, the areas overlying eachother, meaning a common zone, has an approximately similar amount ofmaterial, either by altering one or the other, or both, of the twolayers. The alternation can be in the form of material additions, suchas by deposition, or material subtractions, such as by etching. Theeffect is the same: the package reliability is enhanced by accountingfor CTE gradients across the length and width of the individual,symmetrically matched layers.

IV DIE AREA CTE CONTROL

Referring again to FIGS. 1 and 2, an integrated circuit chip, such aschip 18, is typically made of a material, such as silicon, that has asubstantially different coefficient of thermal expansion (CTE) ascompared to an underlying package made of non-ceramic materials, such asthe package 12. These differences in CTE apply stress to the solder ballconnections that attach the chip 18 to the package 12.

A solution to this problem entails making the package have two differentCTEs, one approximately matching that of the printed wiring board (PWB)40 and the other approximately matching that of the chip 18. In otherwords, the CTE characteristics of the chip 18 are matched approximatelyto the CTE of the portion 12A of the substrate 12 that is directly underthe chip 18.

In general, since the chip 18 has a lower CTE than that of organicpackages, the portion 12A of the package will have a lower CTE than thesurrounding area of the substrate. That is not to say, however, that theconditions could not be reversed, where for purposes of mounting otheror different components, the center area could have a higher CTE thanthe surrounding area.

Creating a different CTE for the middle, chip-mounting area 12A of thepackage 12 can be accomplished in a variety of ways. One is to usedifferent materials in the multi-layered structure for the centralregion, such that the materials themselves are chosen to provide adifferent CTE. The combination of layered materials for the portion 12Aof the package 12 that is under the chip 18, e.g., copper, molybdenum orInvar, are selected to produce an overall CTE in the region 12A that issimilar to but generally greater than the CTE of the chip 18 through thenormal range of operating temperatures for both the chip 18 and thepackage 12. In this manner, undesirable relative movement between thechip 18 and the substrate 12 is largely eliminated.

Having approximated the CTE of the portion 12A of the package 12 to theCTE of the chip 18, it has been found, also in accordance with theinvention, that the average CTE of the entire package 12 now should bematched to the CTE of the PWB 40 to which the package 12 and itsattached chip 18 are joined. Failing to essentially approximate the CTEof the PWB 40 with that of the aggregate CTE for the package 12 and thechip 18 can lead to the generation of a thermally created bending andshearing movement between the PWB 40 and the package 12. The unbalancedforces thereby created between the PWB 40 and the package 12 aretransferred to the chip 18 which responds to this curving and relativemovement in the undesirable manner noted above, e.g., shearing thesoldered electrical connections or the electrical connections betweenthe package and the PWB 40. In a similar manner, solder balls 42 used tomake electrical connections between the package 12 and the PWB 40 alsoare subject to undesirable shearing events. In a typical embodiment ofthis aspect of the invention, the following components and theirillustrative CTE characteristics that are to be reconciled are asfollows:

    ______________________________________    Component     CTE (PPM/°C.)    ______________________________________    chip 18       2-3    package 12    ≈20    PWB 40        16-17    ______________________________________

Accordingly, the area 12A of the package 12 under the chip 18 shouldenjoy a CTE as close to the range of 2-3 PPM/°C. as is possible,illustratively 6 to 10 PPM/°C. The aggregate or average CTE of thepackage 12 also should be in the 16-17 PPM/°C. range in order toapproach the printed circuit board CTE. Expressed in mathematical terms:##EQU1## where α=Spatially varying package 12 CTE in the x,y plane; andα=Average CTE for the entire package 12.

The use of a different CTE for a chip-underlying portion of a package isparticularly suited for flip chip packages or others employingperipheral ball grid array connectors between the package and the PWB.As seen in FIG. 2, solder balls are not used between the PWB 40 and thepackage 12 in the area underlying the chip cavity 30. The reason is thatthere will be a CTE mismatch between the area 12A of the package 12, nowlowered to match the CTE of the chip 18. However, some or all of thearea under the chip 18 may be populated with solder balls 42 if therelative movement between the PWB 40 and the chip/package combinationcan be tolerated in the specific application.

While the embodiment illustrated and described above focuses on asubstrate having two discrete regions and two discrete CTEs, severaladditional regions could be encountered, particularly when mountingmultiple chips on the package, as in multi-chip modules (MCMs).Moreover, the chip mounting region does not have to be centrally locatedbut can be disposed virtually in any position on the package.

V SELECTIVE CTE ADJUSTMENT

Where it is desirable to have a different CTE in different regions ofthe package 12, as for instance, when attempting to match the CTE of thepackage to two different components, the CTE can be varied by physicallyaltering the layered structure that forms the package 12.

Referring FIG. 7, a core layer 82 is drilled, etched or otherwise workedto include a plurality of transversely oriented holes 84 which extendthrough the core layer from one surface 86 to the opposite surface 88.The core layer 82 is one that would correspond to the core layer 52 ofFIG. 3, and as with layer 52, the core layer is usually made of copper.

In FIG. 8, the core layer 82 is shown stacked between two dielectriclayers 90 and 92. Typically, a lamination assembly of layers willadditionally include conductive layers 94 and 96 disposed respectivelyon opposite sides of the dielectric layers 90 and 92.

When the dielectric layers are made of an organic material which justbefore pressing is in a b-stage, the dielectric material flows into andfills the holes 84, as shown in FIG. 9. The core layer 82 thus becomes acomposite which includes a copper matrix having dielectric materialdispersed therethrough. The dielectric material has a higher CTE thancopper and thus, the CTE of the core layer is increased where the holes84 are present.

Alternatively, the holes 84 could be filled with any material prior tolamination to achieve a desired effect on the CTE of the package. Forexample, a filler having a lower CTE could fill the holes 84 as a liquidand solidified before passing the core 82 on the lamination step.

As a further alternative, and referring to FIG. 10, a core layer 98 canbe machined, etched or otherwise worked to form one or more grooves 100which are filled with a filler material 102 which is selected to achievea desired CTE change. A lower CTE filler will lower the CTE of the core98, and a higher CTE will raise the core CTE.

Whether holes, grooves or other formations are used, the core can beworked with any number of known techniques, such as laser drilling,machining, punching, etching, etc. Moreover, the formations are notnecessarily made to the core layer of a laminated package, but could bemade in any layer, whether dielectric or conductive.

When making formations in layers beyond the core layers, the symmetriccounterpart layers must be similarly modified. For example, in FIG. 3,if holes are to made and filled in the conductive layer 64, the samepattern of holes and same filler must be used in the same position ofthe symmetrically opposed conductive layer 70.

As seen in FIG. 11, the holes 84 can be arranged in a grid-like patternin the core layer 82 in a relatively tightly packed arrangement wherelocalized changes in CTE are required. For example, to match the CTE ofthe package 12 in an area under the chip 18 (FIG. 2), the core layer 82can be patterned with holes in the chip mounting area that overlies thecore layer. The holes are then filled with a material that increases ordecreases the CTE of the package in that area.

VI CHIP PACKAGE LID CTE ADJUSTMENT

As mentioned above, the chip 18, which can be on the order of 1 cm² to 4cm² generates an unusual amount of heat, on occasion as much as 100watts. To avoid degrading or even destroying the electricalcharacteristics of the chip 18, it is necessary to provide some meansfor dissipating this heat to enable the chip 18 to continue operationwithin an acceptable temperature.

To provide this important function of dispersing heat generated by thechip 18 to the lid 32 for subsequent dissipation, in accordance withthis aspect of the invention, it has been found that using a paste ofthe type described in the Ameen et al. '473 patent and the pastedielectrics described herein provide a suitable thermally conductive,CTE matching adhesive in the volume for joining the chip 18 to the lid32.

Another way, in accordance with a further principal of the invention,for approximating the CTE of the lid 32, in turn, to the CTE of the chip18 and to the considerably different CTE of the constraining ring 22 andpackage 12 in order to reduce the degree of bending caused by mismatchedCTE characteristics is through a manipulation of the materialcomposition of the lid 32.

As mentioned above, prior art lids usually were formed from eitheraluminum or copper or a composite material, e.g., by adding eitheraluminum or copper to silicon carbide or some other low CTEreinforcement matrix.

Aluminum, for instance, has a CTE of 23 PPM/°C. and pure silicon carbidehas a CTE of 3.7 PPM/°C. Consequently, the potential exists for the lid32 to enjoy a predetermined CTE anywhere in the range between 3.7PPM/°C. for pure silicon carbide to 23 PPM/°C. for pure aluminum.

In accordance with the present invention it has been determined that thelid should be provided with different regions of CTE to match thedifferent components of the chip/package 10 to which the lid 32 is inthermal and physical contact. This can be accomplished in a variety ofways. For example, the lid 32 can be made of a metal matrix materialwhich includes silicon carbide and aluminum. The concentration ofmaterials can be varied to achieve a desired two-region CTE in the lid,and in particular, the central region of the lid, correspondingpositionally to the chip 18, will be configured to have a lower CTE thanthe surrounding region of the lid 32, which preferably is CTE matched tothe higher CTE constraining ring 22.

A two-region CTE lid can be constructed in the following manner.Referring to FIG. 12, a preform 104 is made of silicon carbide powder orwhiskers. The preform can be pressed to achieve a desired shaped, or canbe machined or cut to form its desired shape. The desired shape is onewhere the middle region 106 has a thicker dimension than the outerregions 108 and 110. The preform 104 can be made using standard powdermetallurgy techniques.

The preform 106 is next placed in a mold and pressure infiltrated withmolten aluminum. As seen in FIG. 13, the mold die 112 and 114approximately abut the middle region of the preform, and form voidsaround the outer regions. After pressure infiltration, and as shown inFIG. 14, a finished lid 116 has the overall shape desired to fit thepackage 12, but since the middle region has a higher concentration ofsilicon carbide, the middle region will have a lower CTE. Specifically,the lower CTE is designed to approximately match that of the chip, whichis about 2.6, while the outer regions having higher concentrations ofhigher CTE aluminum, approximately matches that of the constrainingring.

Because the peripheral portion of the lid that is bonded to theconstraining ring should have a high aluminum and low silicon carbideconcentration to approach the CTE for the ring, in absorbing thealuminum in the peripheral portion of the silicon carbide that forms thelid, relatively more aluminum is added to this peripheral portion thanto the central portion of the lid. By selecting the relativeconcentrations in respective portions of the lid, the average CTE of thelid matches the average CTE of the package 12 and the different averageCTE of the chip 18. This manipulation of relative aluminum and siliconcarbide concentrations in the lid permits the microchip package andcomponents to remain essentially flat while reducing the stress appliedto the die or the adhesive interface.

The large central portion of the silicon carbide preform, which isessentially porous, establishes a ratio of aluminum to silicon carbidein the different regions of the finished lid. As noted, a greaterconcentration of aluminum relative to the silicon carbide is required inthe vicinity of the ring. These concentrations are established bydissolving aluminum in the preform and filling the voids above and belowthe outer regions of the preform. In this way, a concentration ofaluminum and silicon carbide producing a CTE that approaches the CTE ofthe ring in one region and the chip in another region is provided.

As a result, a technique now is available through the practice of theinvention that enables the CTE of certain portions of the lid to beestablished to essentially match the CTE characteristics of othercomponents of the chip/package system 10 (FIG. 1 that are bonded to thecorresponding portions of the lid.

While silicon carbide/aluminum systems are described above, othermaterials may be used to form lids exhibiting different CTEs indifferent regions. It is also possible to use a single powderedmaterial, ceramic or metal, and vary the CTEs in different regions byapplying different consolidation forces. For example, the middle regioncould have a greater or lesser theoretical density than the outerregions, which could provide a sufficient difference of CTE in somecases. In this case, the powdered materials can be selected and/or mixedattain the desired differential in CTE.

An alternative embodiment is illustrated in FIGS. 15-17, in which a lid118 is made to include a central opening 120. An insert 122 is sized tofit in the opening 120. The insert 122 is made of a material that has aCTE that is different than that of the material used to form the lid118. In particular, if a lower CTE is desired for the middle region ofthe lid, as when the middle region overlies the chip, the insert can bemade of material having a lower CTE than the rest of the lid.

To accomplish the embodiment of FIGS. 15-17, the lid 118 can be made asa single piece and the central opening 120 can be machined out, or theopening can be formed integrally, as when the lid is made from powderedmetal or ceramic. The insert can be formed by the same techniques andmay for example comprise a silicon carbide preform infiltrated withaluminum. At any rate, the CTE of the insert can be customized to matchthat of the chip while the lid 118 CTE can be customized to match thatof the constraining ring.

Also, a higher concentration of silicon carbide in the center region ofthe lid can be accomplished by removing the center region of a lowdensity porous silicon carbide preform which has the same dimensions ofthe desired lid. Into this central cavity is placed a slug of highdensity porous silicon carbide which fills the cavity. Afterimpregnating this preform with aluminum, a lid with a center region ofhigh silicon carbide concentration and a peripheral region of lowersilicon carbide concentration results.

VII CTE CANCELLATION

Referring now to FIG. 18, a chip 124 is shown mounted on a package 126through solder balls 128. A constraining ring 130 is mounted to theupper surface of the package 126.

Due to the difference in CTE between the chip 124 and the package 126, abending moment is generated that acts upon the chip 124 and canpotentially compromise the flatness of the chip. The differences in CTEsare largely due to the material differences in that the chip 124 issilicon and the package is one which utilizes thin organic dielectriclayers made of the materials described herein.

According to the invention, a false die 132 is bonded to the lowersurface of the package 126 prior to bonding the chip to the package. Thefalse die 132 is CTE matched to the chip 124 and tends to oppose thebending moments that are generated by the chip/package attachment,particularly as the underfill adhesive is cured. Thus, the false die 132counterbalances the moments during chip attach with opposing bendingmoments.

The false die 132 can be made of silicon but other materials exhibitingCTEs similar to the chip 124 can be used. However, it should bepositioned directly opposite to the chip 124. Also, materials havingdifferent CTEs over the silicon chip could be used, if made thicker toresist the bending moments. For example, aluminum has a CTE of about 9,but could be used if made thick enough to get the same effect.

A further aspect of this invention is to adhesively bond the false dieto the package at the same time the underfill adhesive is applied to theregion between the chip 124 and the package 126. The co-curing of bothadhesives will ensure that the false die bending moments offset bendingmoment generated by adhesive shrinkage at the underfill.

While the false die 132 can be a passive stiffener, it can also take theform of an electrical component such as a capacitor formed on thepackage. Also, the false die could be a real chip that failed qualitycontrol or it can be one or more circuit components that are frequentlyreferred to as "passive" components, e.g., capacitors, resistors andinductances. The significant point being, however, that the passivecomponent, throughout the range of anticipated operating temperaturesfor the microchip package, should provide thermal expansion causedbending moments that are essentially equal to, but opposite in directionto, those created by the chip.

In this manner, the forces that otherwise would warp or bend the chipand the substrate being equal but on opposite sides of the substratemutually cancel each other. The physical consequence of the balancebetween these equal and opposing forces is to cancel each other, andthereby enable the chip to remain essentially flat.

The following table is illustrative of the experimentally developedeffects of various materials, used as the electrically passivecomponents.

    ______________________________________    Effect of Different Materials    Deflection, Stresses and Critical Flaw Size    on Cooling from 150° C. to 25° C.    Inactive            Die      Package    Die    Materials    Deflection                                 Deflection                                        σ.sub.1                                              a.sub.c    Material           Properties   (μm)  (μm)                                        (Mpa) (mm)    ______________________________________    Silicon           E = 130 Gpa  -1.1     -62.6  11.7  1.141           v = 0.28           α = 2.6 × 10.sup.-6 °C..sup.-1    96%    E = 320 Gpa  -28.5    -186.2 15.0  0.860    Al.sub.2 O.sub.3           v = 0.2           α = 7.0 × 10.sup.-6 °C..sup.-1    12/85/12           E = 140 Gpa  -13.8    -119.0 12.4  1.259    CIC    v = 0.3           α = 4.4 × 10.sup.-6 °C..sup.-1    Mo     E = 317 Gpa  -14.0    -119.8 13.3  1.094           v = 0.3           α = 5.0 × 10.sup.-6 °C..sup.-1    ______________________________________

Illustrative of the improvement afforded by different stiffenermaterials to maintain die area and microchip package flatness, attentionis invited to the following table:

    ______________________________________    Effect of Different Die Area Stiffeners (prior to chip attach)    Stiffener Material                 Maximum Die   Maximum Package    and Thickness                 Non-Planarity Non-Planarity    ______________________________________    381    μm Copper                     -9.7     μm  -107   μm    254    μm Copper                     -11.4    μm  -108   μm    71     μm Copper                     -14.9    μm  -120   μm    35     μm Copper                     -16.8    μm  -132   μm    381    μm Silicon                     +69.7    μm  -47    μm    71     μm Silicon                     +64.3    μm  -80    μm    381    μm Alumina                     +29.9    μm  -74    μm    71     μm Alumina                     +50.9    μm  -91    μm    ______________________________________

Industrial Utility

Each of the embodiments of the invention described above significantlycontribute to an improved microchip package through the essentialelimination, control or reduction of relative movement among theindividual components that form the microchip package, withoutlimitation to the source of this relative movement. Thus, throughCTE-based warpage, bending due to manufacturing tolerances in thedifferent substrate layers, or the like, each of these sources ofrelative movement among the microchip package components and theundesirable consequence of such movement, is alleviated to a significantextent.

As discussed above, the adhesives 28 and 38 can be a variety of adhesivematerials. In one embodiment, adhesives include a fluoropolymermaterial, such as, is a porous polytetrafluoroethylene (PTFE), andespecially an extruded and/or expanded PTFE, such as that taught in U.S.Pat. No. 3,953,566 to Gore. The material is usually porous expanded PTFEwhich has been stretched at least 2 to 4 times its original size inaccordance with U.S. Pat. No. 3,953,566. This stretching created poresthat act as natural air reliefs when the filled material is urgedbetween two components. In addition, because of the nature of theexpanded PTFE, stresses created due to mismatches in the thermalcoefficients of expansion between component parts may be relieved inthis conductive layer if it placed between them.

Suitable interface compositions include PTFE with about 50 to 60% byvolume of the solid components of ZnO, BN, or any other good thermallybut electrically non-conductive filler. The final product may beexpanded in ratios of 4:1 or 3:1 or 2:1 to achieve the desired degree ofconformability. As has been noted, the presence of the pores createdfrom the expansion process is responsible for the conforming nature ofthe finished product and aids in the relief of trapped air when thismaterial is placed between two parallel plates and then are urgedtogether. These materials may be formed into any suitable shape, such asthin tapes having thicknesses in the 5 to 15 mil (0.127 to 0.381 mm)range.

Another suitable composition for use in the present invention involvesfilling the PTFE with a metal powder, such as copper or nickel, havingparticle sizes in the 1 to 40 micron range. Bimodal and trimodaldistributions can increase the loading of this material, such asproviding particles in the 1 to 5 micron range mixed with particles inthe 40 to 45 micron range. This allows greater packing density, with asubsequent increase in thermal conductivity without sacrificingconformability. The total volume percent (including air) of metal tofinished filled PTFE is in the 20 to 90% range. The finished material 17and 56 can be a material that is a porous matrix system which is imbibedor impregnated an adhesive-filler mixture.

In another embodiment of the present invention, the adhesives 17 and 56are a porous matrix that is a non-woven substrate imbibed with highquantities of filler and a thermoplastic or thermoset adhesive, as aresult of the initial void volume of the substrate, heated to partiallycure the adhesive and form a B-stage composite. Substrates includefluoropolymers, such as the porous expanded polytetrafluoroethylenematerial of U.S. Pat. Nos. 3,953,566 and 4,482,516, each of which isincorporated herein by reference. Desirably, the mean flow pore size(MFPS) should be between about 2 to 5 times or above that of the largestparticulate, with a MFPS of greater than about 2.4 times that of thefiller being particularly preferred. However, it is also within thescope of the invention that suitable composites can be prepared byselecting the ratio of the mean flow pore size to average particle sizeto be greater than 1.4. Acceptable composites can also be prepared whenratio of the minimum pore size to average particle size is at leastabove 0.8, or the ratio of the minimum pore size to the maximum particlesize is at least above 0.4. The MFPS to particle size ratios may bedetermined with a Microtrak® Model FRA Particle Analyzer device.

Alternatively, another mechanism for gauging relative pore and particlesizes may be calculated as the smallest pore size being not less thanabout 1.4 times the largest particle size.

In addition to expanded fluoropolymer substrates, porous expandedpolyolefins, such as ultra high molecular weight (UHMW) polyethylene,expanded polypropylene, polytetrafluoroethylene made prepared by pasteextrusion and incorporating sacrificial fillers, porous inorganic ororganic foams, or microporous cellulose acetate, can also be used.

The porous substrate has an initial void volume of at least 30%,preferably at least 50%, and most preferably at least 70%, andfacilitates the impregnation of thermoset or thermoplastic adhesiveresin and particulate filler paste in the voids while providing aflexible reinforcement to prevent brittleness of the overall compositeand settling of the particles.

The filler comprises a collection of particles when analyzed by aMicrotrak® Model FRA Particle Analyzer device, which displays a maximumparticle size, a minimum particle size and an average particle size byway of a histogram.

Suitable fillers to be incorporated into the adhesive include, but arenot limited to, BaTiO₃, SiO₂, Al₂ O₃, ZnO, ZrO₂, TiO₂, precipitated andsol-gel ceramics, such as silica, titania and alumina, non-conductivecarbon (carbon black) and mixtures thereof. Especially preferred fillersare SiO₂, ZrO₂, TiO₂ alone or in combination with non-conductive carbon.Most preferred fillers include filler made by the vapor metal combustionprocess taught in U.S. Pat. No. 4,705,762, such as, but not limited tosilicon, titanium and aluminum to produced silica, titania, and aluminaparticles that are solid in nature, i.e., not a hollow sphere, with auniform surface curvature and a high degree of sphericity.

The fillers may be treated by well-known techniques that render thefiller hydrophobic by silylating agents and/or agents reactive to theadhesive matrix, such as by using coupling agents. Suitable couplingagents include, silanes, titanates, zirconates, and aluminates. Suitablesilylating agents may include, but are not limited to, functionalsilylating agents, silazanes, silanols, siloxanes. Suitable silazanes,include, but are not limited to, hexamethyidisilazane (Huls H730) andhexamethylcyclotrisilazane, silylamides such as,bis(trimethylsilyl)acetamide (Huls B2500), silylureas such astrimethylsilylurea, and silylmidazoles such as trimethylsilylimidazole.

Titanate coupling agents are exemplified by the tetra alkyl type,monoalkoxy type, coordinate type, chelate type, quaternary salt type,neoalkoxy type, cyclo-heteroatom type. Preferred titanates include,tetra alkyl titanates, Tyzor® TOT {tetrakis(2-ethyl-hexyl) titanate,Tyzor® TPT {tetraisopropyl titanate}, chelated titanates, Tyzor® GBA{titanium acetylacetylacetonate}, Tyzor® DC {titaniumethylacetacetonate}, Tyzor® CLA {proprietary to DuPont}, Monoalkoxy(Ken-React® KR TTS), Ken-React®, KR-55 tetra (2,2diallyloxymethyl)butyl, di(ditridecyl)phosphito titanate, LICA® 38neopentyl(diallyl)oxy, tri(dioctyl)-pyrophosphato titanate. Suitablezirconates include, any of the zirconates detailed at page 22 in theKenrich catalog, in particular KZ 55-tetra (2,2 diallyloxymethyl)butyl,di(ditridecyl)-phosphito zirconate, NZ-01-neopentyl(diallyl)oxy,trineodecanoyl zirconate, NZ-09-neopentyl-(diallyl)oxy,tri(dodecyl)benzene-sulfonyl zirconate.

The aluminates that can be used in the present invention include, butare not limited to Kenrich®, diisobutyl(oleyl)acetoacetylaluminate (KA301), diisopropyl(oleyl)acetoacetyl aluminate (KA 322) and KA 489.

In addition to the above, certain polymers, such as, cross-linkedvinylic polymers, e.g., divinylbenzene, divinyl pyridine or a sizing ofany of the disclosed thermosetting matrix adhesives that are firstapplied at very high dilution (0.1 up to 1.0% solution in MEK) can beused. Also, certain organic peroxides, such as, dicumylperoxide can bereacted with the fillers.

The adhesive itself may be a thermoset or thermoplastic and can includepolyglycidyl ether, polycyanurate, polyisocyanate, bis-triazine resins,poly (bis-maleimide), norbornene-terminated polyimide, polynorbornene,acetylene-terminated polyimide, polybutadiene and functionalizedcopolymers thereof, cyclic olefinic polycyclobutene, polysiloxanes, polysisqualoxane, functionalized polyphenylene ether, polyacrylate, novolakpolymers and copolymers, fluoropolymers and copolymers, melaminepolymers and copolymers, poly(bis phenycyclobutane), and blends orprepolymers thereof. It should be understood that the aforementionedadhesives may themselves be blended together or blended with otherpolymers or additives, so as to impact flame retardancy or enhancedtoughness.

As used herein, mean flow pore size and minimum pore size weredetermined using the Coulter® Porometer II (Coulter Electronics Ltd.,Luton UK) which reports the value directly. Average particle size andlargest particle size were determined using a Microtrak® lightscattering particle size analyzer Model No. FRA (Microtrak Division ofLeeds & Northup, North Wales, P.A., USA). The average particle size(APS) is defined as the value at which 50% of the particles are larger.The largest particle size (LPS) is defined as the largest detectableparticle on a Microtrak® histogram. Alternatively, the largest particlesize is defined at the minimum point when the Microtrak FRA determinesthat 100% of the particulate have passed.

In general, the method for preparing the adhesive-filler compositeinvolves:(a) expanding a polytetrafluoroethylene sheet by stretching alubricated extruded perform to a microstructure sufficient to allowsmall particles and adhesives to free flow into the void or pore volume;(b) forming a paste from polymeric, e.g., thermoset or thermoplasticmaterial and a filler; and (c) imbibing by dipping, coating, pressurefeeding, the adhesive-filler paste into the highly porous scaffold, suchas expanded polytetrafluoroethylene.

Table 1 shows the effect of the relationship of the substrate mean flowpore size (MFPS) and particulate size. When the ratio of the mean flowpore size (MFPS) to largest particulate is 1.4 or less, poor results areobserved. In this case, a homogeneous composite is not observed, andmost of the particulate filler does not uniformly penetrate themicroporous substrate. When the ratio of the MFPS to largest particulateis greater than about 2.0, then a uniform composite is obtained. It isalso observed that the larger the ratio of MFPS to largest particulate,the greater the relative case it is to imbibe a homogeneous dispersioninto the microporous substrate.

                  TABLE 1    ______________________________________    Substrate    Pore Size   Particle Size                          MFPS    Pore.sub.Min                                        Pore.sub.Min    Sam- Min    MFPS    Avg  Max  ÷ ÷ ÷ Re-    ple  (μm)                (μm) (μm)                             (μm)                                  Part.sub.Avg                                        Part.sub.Max                                              Par.sub.Avg                                                    sult    ______________________________________    A     4     7       5    10   1.4   0.4   0.8   Poor    B     4     5       5    10   1.0   0.4   0.8   Poor    C    --     58      5    10   12.4  N/A   --    Good    D    18     32      6    10   5.3   1.8   3.0   Good    E    18     32      1    1    32.0  18.0  18    Good    F    17     24      6    10   4.0   1.7   2.8   Good    G    0.2    0.4     0.5  1.6  0.8   0.125 0.4   Poor    H    --     60      18   30   3.3   --    --    Good    I    14     11      0.5  1.6  22.0  8.8   28    Good    J    14     29      4    8    7.3   1.8   3.5   Good    K    14     29      5    10   5.8   1.4   2.8   Good    ______________________________________

EXAMPLE 1

A fine dispersion was prepared by mixing 281.6 g TiO₂ (TI Pure R-900, DuPont Company) into a 20% (w/w) solution of a flame retardeddicyanamide/2-methylimidazole catalyzed bisphenol-A based polyglycidylether (Nelco N-4002-5, Nelco Corp.) in MEK. The dispersion wasconstantly agitated so as to insure uniformity. A swatch of expandedPTFE was then dipped into the resin mixture. The web was dried at 165°C. for 1 min. under tension to afford a flexible composite. Thepartially-cured adhesive composite thus produced comprised of 57 weightpercent TiO₂, 13 weight percent PTFE and 30 weight percent epoxyadhesive. Several plies of the adhesive sheet were laid up betweencopper foil and pressed at 600 psi in a vacuum-assisted hydraulic pressat temperature of 225° C. for 90 min. then cooled under pressure. Thisresulted in a copper laminate having dielectric constant of 19.0, andwithstood a 30 sec. solder shock at 280° C. at an average ply thicknessof 100 mm (0.0039"(3.9 mil)) dielectric laminate thickness.

EXAMPLE 2

A fine dispersion was prepared by mixing 386 g SiO₂ (HW-11-89, HarbisonWalker Corp.) which was pretreated with phenyltrimethoxysilane (04330,Huls/Petrarch) into a manganese catalyzed solution of 200 g bismaleimidetriazine resin (BT2060BJ, Mitsubishi Gas Chemical) and 388 g MEK. Thedispersion was constantly agitated so as to insure uniformity. A swatchof 0.0002" thick expanded PTFE was then dipped into the resin mixture,removed, and then dried at 165° C. for 1 min. under tension to afford aflexible composite. Several plies of this prepreg were laid up betweencopper foil and pressed at 250 psi in a vacuum-assisted hydraulic pressat temperature of 225° C. for 90 min. then cooled under pressure. Thisresulting dielectric thus produced comprised of 53 weight percent SiO₂,5 weight percent PTFE and 42 weight percent adhesive, displayed goodadhesion to copper, dielectric constant (at 10 GHz) of 3.3 anddissipation factor (at 10 GHz) of 0.005.

EXAMPLE 3

A fine dispersion was prepared by mixing 483 g SiO₂ (HW-11-89) into amanganese-catalyzed solution of 274.7 g bismaleimide triazine resin(BT2060BJ, Mitsubishi Gas Chemical) and 485 g MEK. The dispersion wasconstantly agitated so as to insure uniformity. A swatch of 0.0002"thick expanded PTFE was then dipped into the resin mixture, removed, andthen dried at 165° C. for 1 min. under tension to afford a flexiblecomposite. Several plies of this prepreg were laid up between copperfoil and pressed at 250 psi in a vacuum-assisted hydraulic press attemperature of 225° C. for 90 minutes then cooled under pressure. Theresulting dielectric thus produced comprised of 57 weight percent SiO₂,4 weight percent PTFE and 39 weight percent adhesive, displayed goodadhesion to copper, dielectric constant (at 10 GHz) of 3.2 anddissipation factor (at 10 GHz) of 0.005.

EXAMPLE 4

A fine dispersion was prepared by mixing 15.44 kg TiO₂ powder (TI PureR-900, DuPont Company) into a manganese-catalyzed solution of 3.30 kgbismaleimide triazine resin (BT2060BH, Mitsubishi Gas Chemical) and15.38 kg MEK. The dispersion was constantly agitated so as to insureuniformity. A swatch of 0.0004" TiO₂ -filled expanded PTFE (filledaccording to the teachings of Mortimer U.S. Pat. No. 4,985,296, exceptto 40% loading of TiO₂ and the membrane was not compressed at the end)was then dipped into the resin mixture, removed, and then dried at 165°C. for 1 min. under tension to afford a flexible composite. Thepartially cured adhesive composite thus produced comprised of 70 weightpercent TiO₂, 9 weight percent PTFE and 21 weight percent adhesive.Several plies of this prepreg were laid up between copper foil andpressed at 500 psi in a vacuum-assisted hydraulic press at temperatureof 220° C. for 90 minutes then cooled under pressure. This resultingdielectric displayed good adhesion to copper, dielectric constant of10.0 and dissipation factor of 0.008.

EXAMPLE 5

A fine dispersion was prepared by mixing 7.35 kg SiO₂ (ADMATECHS SO-E2,Tatsumori LTD) with 7.35 kg MEK and 73.5 g of coupling agent,i.e.,3-glycidyloxypropyltri-methoxysilane (Dynasylan GLYMO (PetrachSystems). SO-E2 is described by the manufacture as having highlyspherical silica having a particle diameter of 0.4 to 0.6 mm, a specificsurface area of 4-8 m² /g, a bulk density of 0.2-0.4 g/cc (loose).

To this dispersion was added 932 g of a 50% (w/w) solution of a cyanatedphenolic resin, Primaset PT-30 (Lonza Corp.). In (MEK)methylethylketone, 896 g of a 50% (w/w) solution of RSL 1462 (ShellResins, Inc.(CAS #25068-38-6)) in MEK, 380 g of a 50% (w/w) solution ofBC-58 (Great Lakes, Inc.) in MEK, 54 g of 50% solution of bisphenol A(Aldrich Company) in MEK, 12.6 g Irganox 1010 (Ciba Geigy), 3.1 g of a0.6% solution of Manganese 2-ethylhexanoate (Mn HEX-CEM (OMG Ltd.), and2.40 kg MEK. This dispersion was subjected to ultrasonic agitationthrough a Misonics continuous flow cell for about 20 minutes at a rateof about 1-3 gal./minute. The fine dispersion thus obtained was furtherdiluted to an overall bath concentration of 11.9% solids (w/w).

The fine dispersion was poured into an impregnation bath. A expandedpolytetrafluoroethylene web having the node fibril structure of FIGS. 19and 20, and the following properties:

    ______________________________________    Frazier               20.55    Coverage              9 g/m.sup.2    Ball Burst            3.2 lbs.    Thickness             6.5 mil.    Mean Flow Pore Size   9.0 microns    ______________________________________

The Frazier number relates to the air permeability of the material beingassayed. Air permeability is measured by clamping the web in a gasketedfixture which is provided in circular area of approximately 6 squareinches for air flow measurement. The upstream side was connected to aflow meter in line with a source of dry compressed air. The downstreamside of the sample fixture was open to the atmosphere. Testing isaccomplished by applying a pressure of 0.5 inches of water to theupstream side of the sample and recording the flow rate of the airpassing through the in-line flowmeter (a ball-float rotameter that wasconnected to a flow meter.

The Ball Burst Strength is a test that measures the relative strength ofsamples by determining the maximum at break. The web is challenged witha 1 inch diameter ball while being clamped between two plates. TheChatillon, Force Gauge Ball/Burst Test was used. The media is placedtaut in the measuring device and pressure affixed by raising the webinto contact with the ball of the burst probe. Pressure at break isrecorded.

The web described above was passed through a constantly agitatedimpregnation bath at a speed at or about 3 ft./min, so as to insureuniformity. The impregnated web is immediately passed through a heatedoven to remove all or nearly all the solvent, and is collected on aroll.

Several plies of this prepeg were laid up between copper foil andpressed at 200 psi in a vacuum-assisted hydraulic press at temperatureof 220° C. for 90 minutes and then cooled under pressure. This resultingdielectric displayed good adhesion to copper, dielectric constant (10GHz) of 3.0 and dissipation factor of 0.0085 (10 GHz).

The physical properties of the particulate filler used in Example 4 andExample 7 are compared below.

    ______________________________________    Property   Tatsumori (ADMATECHS)                               Harbison Walker    ______________________________________    Manufacture               Vapor Metal Combustion                               Amorphous Fused    Technique                  Silica    Designation               Silica SO-E2    HW-11-89    Median Particle Size               0.5 micron      5 micron    Shape      Spherical       Irregular, jagged    Surface Area               6-10 m.sup.2 /g 10 m.sup.2 /g    Bulk Density               0.47 g/cc       1.12 g/cc    Specific Density               2.26 g/cc       2.16 g/cc    ______________________________________

EXAMPLE 6

An ePTFE matrix containing an impregnated adhesive filler mixture, basedon SiO₂ prepared from the vapor combustion of molten silicon is preparedas follows. Two precursor mixtures were initially prepared. One being inthe form of a slurry containing a silane treated silica similar to thatof Example 5 and the other an uncatalyzed blend of the resin and othercomponents.

Mixture I

The silica slurry is a 50/50 blend of the SO-E2 silica of Example 5 inMEK, where the silica contains a coated of silane which is equal to 1%of the silica weight. To a five gallon container, 17.5 pounds of MEK and79 grams of silane were added and the two components mixed to ensureuniform dispersion of the silane in the MEK. Then, 17.5 pounds of thesilica of Example 5 were added. Two five gallon containers of theMEK-silica-silane mixture were added to a reaction vessel, and thecontents, i.e., the slurry, was recirculated through an ultrasonicdisperser for approximately one hour to break up any silica agglomeratesthat may be present. The sonication was completed and the contents ofthe reaction vessel were heated to approximately 80° C. forapproximately one hour, while the contents were continuously mixed. Thereacted mixture was then transferred into a ten gallon container.

Mixture II

The desired resin blend product is an MEK based mixture containing anuncatalyzed resin blend (the adhesive) contains approximately 60%solids, where the solid portion is an exact mixture of 41.2% PT-30cyanated phenolic resin, 39.5% RSL 1462 epoxy resin, 16.7% BC58 flameretardant, 1.5% Irganox 1010 stabilizer, and 1% bisphenol A co-catalyst,all percentages by weight.

Into a ten gallon container, 14.8 pounds of PT-30 and 15-20 pounds ofMEK were added and stirred vigorously to completely solvate the PT-30.Then 6 pounds of BC58 were measured and added to the MEK/PT-30 solutionand vigorously agitated to solvate the BC58. The stabilizer, 244.5 gramsof Irganox 1010 and bisphenol A, 163 grams were added. The ten galloncontainer was reweighed and 14.22 pounds of RSL 1462 were added.Additional MEK was added to bring the mixture weight to 60 pounds. Thecontents were then vigorously agitated for approximately 1 to 2 hours,or as long is necessary to completely dissolve the solid components.

The desired product is a mixture of the silica treated with a silane,the uncatalyzed resin blend, and MEK in which 68% by weight of thesolids are silica, and the total solids are between 5% and 50% by weightof the mixture. The exact solids concentration varies from run to run,and depends in part on the membrane to be impregnated. The catalystlevel is 10 ppm relative to the sum of the PT-30 and RSL1462.

The solid contents of mixtures I and II were determined to verify theaccuracy of the precursors and compensate for any solvent flash that hadoccurred. Then mixture I was added to a ten gallon container to provide12 pounds of solids, e.g., 515 solids content, 23.48 pounds of mixtureI. Then mixture II was added to the container to provide 5.64 pounds ofsolids, e.g., 59.6% solids, 9.46 pounds of mixture II. the manganesecatalyst solution (0.6% in mineral spirits), 3.45 grams, was added tothe mixture of mixture I and mixture II and blended thoroughly to form ahigh solids content mixture.

The bath mixture for impregnating an ePTFE matrix, 28% solids mixture,was prepared by adding sufficient MEK to the high solids content mixtureto a total weight of 63 pounds.

Thereafter, an ePTFE matrix was impregnated with this bath mixture toform a dielectric material.

EXAMPLE 7

A fine dispersion was prepared by mixing 26.8 grams Furnace Black(Special Schwarz 100, Degussa Corp., Ridgefield Park, N.J.) and 79 gramsof coupling agent (Dynaslan GLYMO CAS #2530-83-8;3-glycidyloxypropyl-trimethoxysilane (Petrach Systems). The dispersionwas subjected to ultrasonic agitation for 1 minute, then added to astirring dispersion of 17.5 pounds SiO₂ (SO-E2) in 17.5 pounds MEK whichhad previously been ultrasonically agitated. The final dispersion washeated with constant overhead mixing for 1 hour at reflux, then allowedto cool to room temperature.

Separately, an adhesive varnish was prepared by adding the following:3413 grams of a 57.5% (w/w) mixture of Primaset PT-30 in MEK, 2456 gramsof a 76.8% (w/w/) mixture of RSL 1462 in MEK, 1495 grams of a 53.2%(w/w) solution of BC58 (Great Lakes, Inc.) in MEK, 200 grams of 23.9%(wlw) solution of bisphenol A (Aldrich Company) in MEK, 71.5 gramsIrganox 1010, 3.21 grams of a 0.6% (w/w) solution of Mu HEX-CEM (OMGLtd.) in mineral spirits, and 2.40 kg MEK.

In a separate container, 3739 grams of the dispersion described abovewas added, along with 0.0233 grams of Furnace Black (Special Schwarz100, Degussa Corp., Ridgefield Park, N.J.), 1328 of the adhesive varnishdescribed above and 38.3 pounds MEK. This mixture was poured into animpregnation bath, and an ePTFE web was passed through the impregnationbath at a speed at or about 3 ft/min. This dispersion was constantlyagitated so as to insure uniformity. The impregnated web is immediatelypassed through a heated oven to remove all or nearly all the solvent,and is collected on a roll.

Several piles of this prepeg were laid up between copper foil andpressed at 200 psi in a vacuum-assisted hydraulic press at temperaturesof 200° C. for 90 minutes then cooled under pressure. This resultingdielectric displayed good adhesion to copper.

EXAMPLE 8

An adhesive varnish was prepared by adding the following: 3413 grams ofa 57.5% (w/w) solution of Primaset PT-30 (PMN P-88-1591)) in MEK, 2456grams of a 76.8% (w/w) solution of RSL 1462 in MEK, 1495 grams of a53.2% (w/w) solution of BC58 (Great Lakes, Inc.) in MEK, 200 grams of23.9% (w/w) solution of bisphenol A (Aldrich Company) in MEK, 71.5 gramsIrganox 1010, 3.21 grams of a 0.6% (w/w) solution of Mn HEX-CEM inmineral spirits, and 2.40 kg MEK.

In a separate container, 1328 grams of the adhesive varnish describedabove, 42.3 pounds MEK, 6.40 grams of Furnace Black (Special Schwarz100, Degussa Corp., Ridgefield, N.J.) and 1860.9 grams SiO² (SO-E2).This mixture was poured into an impregnation bath, and an ePTFE web waspassed through the impregnation bath at a speed at or about 3 ft/min.The dispersion was constantly agitated so as to insure uniformity. Theimpregnated web is immediately passed through a heated oven to removeall or nearly all the solvent, and is collected on a roll.

Several piles of this prepeg were laid up between copper foil andpressed at 200 psi in a vacuum-assisted hydraulic press at temperatureof 220° C. for 90 minutes then cooled under pressure. This resultingdielectric displayed good adhesion to copper.

Although the invention has been described in conjunction with specificembodiments, it is evident that many alternatives and variations will beapparent to those skilled in the art in light of the foregoingdescription and annexed drawings. Accordingly, the invention is intendedto embrace all of the alternatives and variations that fall within thespirit and scope of the appended claims.

What is claimed is:
 1. A method of minimizing warp and die stress in anelectronic assembly comprising the steps of:providing a die; providing alaminated substrate having an organic dielectric material, saidlaminated substrate having a size, shape and coefficient of thermalexpansion (CTE); providing a structural member having a size, shape andcoefficient of thermal expansion (CTE); connecting one surface of thedie to the laminated substrate; connecting an opposite surface of thedie to the structural member; and allowing the die to undergo thermalchanges by operation of the die; the structural member providingthermally caused bending moments that are substantially equal to, but inan opposite direction to, thermally caused bending moments generated bythe die.
 2. A method according to claim 1, wherein the step ofconnecting one surface of the die to the laminated substrate includesadhesively bonding one surface of the die to said laminated substrate.3. A method according to claim 2, wherein the step of connecting theopposite surface of the die to the structural member includes bonding aconstraining ring having a central opening to the package with thecentral opening positioned around the die and defining a die cavity,bonding the structural member to constraining ring over the die, andinjecting an adhesive into the die cavity to provide an adhesive bondbetween at least the opposite surfaces of the die respectively to thestructural member and the package.
 4. A method according to claim 3,further comprising forming at least one opening in at least one of thestructural member and the laminated substrate as an ingress for injectedadhesive.
 5. A method according to claim 3, further comprisingsubstantially filling the cavity with adhesive.
 6. A method according toclaim 1, further comprising the step of connecting a constraining ringto the laminated substrate, wherein the structural member is a lidconnected to the constraining ring over the die.
 7. A method accordingto claim 1, wherein the structural member and the laminated substratehave substantially matching CTEs.
 8. A method according to claim 1,wherein the step of connecting the opposite surface includes applying anadhesive material between the opposite surface of the die and thestructural member.
 9. A method according to claim 8, wherein theadhesive material is a thermally conductive adhesive.